In a default installation of LM 17.x the package systemd is not installed. Software, which is not installed, cannot give a security leak and there is obviously nothing to update. Besides that the fixed version of systemd appears here fine in synaptic (Mint 17.3).

Q: Why doesn't systemd 204-5ubuntu20.26 appear in the Update Manager?
A: As Cosmo. rightly pointed out systemd 204-5ubuntu20.26 does not appear in Update Manager because it has not been installed here, either.